As for the level itself, I feel like you used game difficulty the way you did wholly from a lack of understanding of how it works. Basically, to make this level well, you need to massively rework the level in one of two ways: either the Super Meat Boy or VVVVVV style of quick death with frequent checkpoints, nullifying the consequence—Super Hardcore Mari0 is a good example of this, or go for the Dark Souls style of slow, careful gameplay with few checkpoints. The Dark Souls style only works, however, because all and every trap is visible and understandable before it springs, and the player can make up their failures be regaining health. For you to do this, you would need more Mushrooms—perhaps with fire resistant enemies—and very few instant deaths.